<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<script type="text/javascript">
			// int num;
			// 只是声明变量，没有赋值，现在的值是undefined
			var num;
			console.log(num); //undefined
			console.log(typeof(num));//undefined
			num = 3.14;
			console.log(num); //3.14
			console.log(typeof(num));//number
			num = 'js';
			console.log(num); //js
			console.log(typeof(num));//string
			num = true;
			console.log(num); //true
			console.log(typeof(num));//boolean
			num = null;
			console.log(num); //null
			console.log(typeof(num));//object
		</script>
	</head>
	<body>
	</body>
</html>
